public class Circle {
    private double R;
    public Circle() {
        this.R = 0.0;
    }
    public Circle(double R) {
        this.R = R;
    }
    public void setR(double R) {
        if (R >= 0) {
            this.R = R;
        } else {
            System.out.println("false");
        }
    }
    public double getR() {
        return R;
    }
    public double Circumference() {
        return 2 * Math.PI * R;
    }
    public double Area() {
        return Math.PI * R * R;
    }
    public static void main(String[] args) {
        Circle circle = new Circle(1);
        double circumference = circle.Circumference();
        System.out.println("周长: " +circumference);
        double area = circle.Area();
        System.out.println("面积: " +area);
    }
}
